1. Field of the Invention
This invention relates to testing of electrical systems and, more particularly, to devices for arc fault testing of electrical distribution circuits. The invention also relates to a method for arc fault testing of electrical distribution circuits.
2. Background Information
The common type of circuit breaker used for residential, commercial, and light industrial applications has an electromechanical thermal-magnetic trip device to provide an instantaneous trip in response to a short circuit and a delayed trip in response to persistent overcurrent conditions. Some of these circuit breakers include ground fault protection, which trips the ground fault circuit interrupter (GFCI) in response to a line-to-ground fault, and in some cases, a neutral-to-ground fault. Ground fault protection is provided by an electronic circuit which is set to trip at about 4 to 6 mA of ground fault current for people protection, and at about 30 mA for equipment protection. It is known to incorporate a test circuit in the circuit breaker, which tests at least portions of the electronic ground fault trip circuit. It is also known to test for proper wiring connections. Test circuits for this purpose are commercially available.
Recently, there has been rising interest in also protecting such power distribution circuits, and particularly the branch circuits for homes, commercial and light industrial applications, from arcing faults. Arcing faults are intermittent, high impedance faults, which can be caused for instance by worn or damaged insulation, loose connections, broken conductors and the like. Arcing faults can occur in the permanent wiring, at receptacles, or more likely, in the wiring of loads or extension cords plugged into a receptacle. Because of the intermittent and high impedance nature of arcing faults, they do not generate currents of sufficient instantaneous magnitude or sufficient average current to trigger the thermal-magnetic trip device which provides the short circuit and overcurrent protection.
Various types of arc fault detectors have been developed and/or proposed. Generally, the detectors are of two types. One type responds to the random high frequency noise content of the current waveform generated by an arc. This high frequency noise tends to be attenuated, especially by the presence of filters on some loads, which can be connected to the branch circuit. The other basic type of arc fault detector responds to the step increase in current occurring as the arc is repetitively and randomly struck. Examples of arc fault detectors of the latter type are disclosed in U.S. Pat. Nos. 5,224,006; and 5,691,869.
U.S. Pat. No. 5,459,630 discloses several forms of built-in test circuits for arc fault detectors. In one embodiment, in which the arc fault detector utilizes a coil to sense current, the test circuit adds a capacitor which forms with the impedance of the coil an oscillator generating a waveform with an amplitude which simulates the rapid rise of a step change in current produced by an arc. In another embodiment, the user must repetitively close a switch, which connects a resistor between the line conductor and neutral, to again generate large amplitude pulses.
While the built-in arc fault and ground fault testers test the response of the electronic circuits to simulated conditions, they do not necessarily indicate whether the device will adequately respond in a real installation. One difficulty is that the circuit breaker containing the detectors is located at a load center together with the circuit breakers for other circuits in the installation. However, the fault condition can occur anywhere downstream and can be further distanced from the circuit breaker and detectors by an extension cord. The wiring, and particularly the extension cord, can insert considerable resistance between the fault and the detector, which attenuates the signal sensed by the detector. When the effects of this resistance are combined with the low amplitude of the currents generated by these faults, the detectors may not have sufficient sensitivity to detect remote faults. Another problem can arise when a receptacle is not connected as intended.
Detection of an arcing fault is complicated by the fact that some normal loads can produce waveforms similar to arcing faults. Arc fault detectors attempt to distinguish over such phenomena to minimize nuisance faults. The task is further complicated by the fact that, as mentioned above, arcing faults tend to be smaller in amplitude than dead faults. Furthermore, arcing faults tend to be relatively intermittent.
With the introduction of arc fault circuit interrupter (AFCI) devices, such as arc fault circuit breakers, there exists the need for a method and apparatus for determining the location of problems within electrical wiring.
There is also a need for such an apparatus, which is flexible, simple, safe, and economical.
These needs and others are met by the present invention, which enables suitable voltage and current levels to be safely applied to an electrical system having an arcing fault condition without creating a hazardous or unsafe condition.
As one aspect of the invention, a pulsing apparatus for an electrical system comprises: first terminals structured to input a first voltage being about a nominal voltage of the electrical system; means for generating a signal from the first voltage, the signal having a duty cycle; means for transforming the first voltage to a second voltage having the duty cycle; second terminals; and means for outputting a current at about the second voltage to the second terminals, wherein the second voltage is greater than the first voltage and less than a rated voltage of the electrical system, the second voltage being adapted to identify an arcing fault in the electrical system, and wherein the duty cycle limits an average value of the current to less than about 6 mA.
As another aspect of the invention, a method for identifying an arcing fault in an electrical system comprises the steps of: inputting a first voltage, the first voltage being about a nominal voltage of the electrical system; generating a signal from the first voltage, the signal having a duty cycle; transforming the first voltage to a second voltage having the duty cycle, the second voltage being greater than the first voltage and less than a rated voltage of the electrical system, the second voltage being adapted to identify an arcing fault in the electrical system; outputting a current at about the second voltage to the electrical system; and employing the duty cycle to limit an average value of the current to less than about 6 mA.